This invention relates to an improvement in a process for producing an olefin polymer which comprises continuously polymerizing an olefin in the presence of a catalyst composed of a transition metal compound component and an organo-metallic compound component in at least two independent polymerization zones connected in series by a pipeline while feeding the olefin and the catalyst into an upstream zone of polymerization and withdrawing the resulting polymer from a downstream zone of polymerization.
In the present application, the term "polymerization" is used to denote both homopolymerization and copolymerization, and the term "polymer", to denote both a homopolymer and a copolymer.
More specifically, the present invention relates, in the aforesaid process, to the improvement wherein
(1) the polymerization in the upstream polymerization zone is carried out in a liquid medium under conditions for slurry polymerization,
(2) the solid polymer-containing slurry discharged from the upstream polymerization zone is fed into a liquid medium exchanging zone through an upper feed opening (A) in its upper portion, said liquid medium exchanging zone being provided between the upstream polymerization zone and the downstream polymerization zone and connected to these zones by pipelines, and an additional supply of the liquid medium is fed into said exchanging zone through a lower feed opening (B) in the lower portion of said exchanging zone,
(3) the discharge flow from an upper discharge opening (C) in the exchanging zone above the upper feed opening (A) is recycled to the upstream polymerization zone, and the discharge flow from a bottom discharge opening (D) in the exchanging zone below the lower feed opening (B) is fed into the downstream polymerization zone, and
(4) the amount of the liquid medium in the discharge flow from the bottom discharge opening (D) is adjusted to 1 to 8 times the volume of the additional liquid medium fed from the lower feed opening (B), and thus the precipitated solid polymer is discharged from the bottom discharge opening (D) while it is concurrent with, and in contact with, the liquid medium from the lower feed opening (B).
The process for producing an olefin polymer is generally known which comprises continuously polymerizing an olefin in the presence of a catalyst in at least two independent polymerization zones connected in series by a pipeline while feeding the olefin and the catalyst into an upstream zone of polymerization and withdrawing the resulting polymer from a downstream zone of polymerization.
When the polymerization in the upstream polymerization zone is carried out in a liquid medium under slurry polymerization conditions in performing such a process for continuous multi-stage polymerization of olefins, it is the usual practice to feed the solid polymer-containing slurry discharged from the upstream polymerization zone, either as such or after concentrating it, to the downstream polymerization zone and to continue the polymerization in the downstream zone. In the practice of this process, the unreacted olefin, a molecular weight controlling agent such as hydrogen, catalyst ingredients such as organoaluminum compounds or electron donors, etc. which the solid polymer-containing slurry from the upstream polymerization zone may contain are entrained in a feed flow to the downstream polymerization zone and affect the polymerization in the downstream polymerization zone. Frequently, therefore, it is difficult to prescribe the desired polymerization conditions in the downstream polymerization zone. Furthermore, in actual operations, the residence times of the individual catalyst particles in the polymerization zone are difficult to control to a nearly constant value, and are substantially different. This means that there is a distribution in the residence times of the catalyst particles. Such a difference tends to result in degrading or varying the quality of the resulting polyolefin.
The present inventors made extensive investigations in order to develop an improved process which can overcome the aforesaid technical problems of the continuous multi-stage polymerization of olefins. These investigations have led to the discovery that the aforesaid technical problems can be overcome by performing the continuous multi-stage polymerization of olefins under the conditions satisfying a combination of the parameters (1), (2), (3) and (4) described above; that the desired polymerization conditions in the downstream polymerization zone can be prescribed easily by using a simple additional means and an easy operation; and that an olefin polymer having an excellent and uniform quality can be produced with industrial advantage while circumventing the undesirable effects of the polymerization conditions of the upstream polymerization zone. It has also been found that an olefin polymer having the desired molecular weight distribution and/or composition distribution can be produced with good quality reproducibility without any undesirable variation in these distributions.
Investigations of the present inventors have shown that the aforesaid excellent improvement can be achieved when under the conditions (1) and (3) described above, the liquid medium exchange zone meeting the condition (2) is provided, and the amount of the liquid medium in the discharge flow from the exchanging zone and the amount of the additional liquid medium fed to the exchanging zone are specified in accordance with the condition (4), thereby discharging the precipitated solid polymer while it is concurrent with, and in contact with, the additional liquid medium from the exchanging zone.
Japanese Patent Publication No. 31227/1977 (published on Aug. 13, 1977) proposes a technique of removing the defect that in performing one-stage slurry polymerization of propylene using propylene in the liquid state as a liquid medium, the resulting polymer contains large amounts of a boiling heptane-soluble polymer and catalyst residues. This patent document discloses a process for producing a polymer or copolymer of propylene, which comprises polymerizing propylene or copolymerizing propylene with another unsaturated hydrocarbon monomer in liquid propylene as a liquid medium, contacting the polymer slurry withdrawn from a zone of said reaction countercurrently with liquid propylene as an additional liquid medium in a separately provided washing tower, separating and removing from the polymer slurry the propylene polymer or copolymer dissolved in the slurry and the unreacted unsaturated hydrocarbon monomer (mainly the liquid propylene) containing the catalyst having residual activity, and re-using a part or the whole of the unreacted unsaturated hydrocarbon monomer as a starting liquid for the polymerization step. This patent document only refers to one-stage slurry polymerization, and gives no description about multi-stage polymerization. Naturally, therefore, it is silent on the aforesaid troubles in the multi-stage polymerization. Particularly, it does not state that the prescription of the desired polymerization conditions in the downstream polymerization zone is adversely affected by the polymerization conditions in the upstream polymerization zone, much less any information about means for eliminating such a trouble. In other words, this patent document fails to give any disclosure about the combination of the parameters ( 1) to (4) in accordance with the present invention and any information serving to lead to these parameters. On the contrary, it only discloses conditions inconsistent with the aforesaid combination of the parameters (1) to (4). For example, this patent document teaches that it is essential to contact the liquid propylene as an additional liquid medium fed from the bottom of the washing tower countercurrently with the polymer slurry withdrawn from the polymerization zone and to feed it into the upper portion of the washing tower.